I recall when that was making its rounds originally. I worked in a newspaper office, and the press service sent the story along with a print of the particular frame people claimed showed the ghost of a boy. It was some sort of reflection or other such mundane explanation; I don't recall the particulars, as that was so long ago.
From Wikipedia: **SPOILERS of URBAN LEGEND**
In the final cut of the movie, there is a scene, just over an hour into the film, in which Jack Holden (Ted Danson) and his mother (Celeste Holm) walk through the house with the baby. As they do so, they pass a background window on the lefthand side of the screen, and a black outline that appears to resemble a rifle pointed down can be seen behind the curtains. As the characters walk back past the window 40 seconds later, a human figure can be seen in that window. A persistent urban legend began circulating August 1990 (shortly before the film's sequel, Three Men and a Little Lady, premiered) that this was the ghost of a boy who had been killed in the house where the movie was filmed. The most common version of this rumor was that a nine-year-old boy committed suicide with a shotgun there, which is inspired by the earlier shape. This notion was discussed on the first episode of TV Land: Myths and Legends in January 2007 and was referenced on an episode of Family Guy, on "Hollywood Babylon", and on an episode of the TV show Supernatural. A variation of the legend states that the ghost was Eric Clapton's son Conor, who died of a fall from a 53rd story window in 1991, over three years after the movie was released. The variation states that Clapton allowed the movie to be filmed in his New York City condominium.
However, according to Snopes.com, a website dedicated to investigating urban legends, the figure is a cardboard cutout "standee" of Jack, wearing a tuxedo and top hat, that was left on the set. This prop was created as part of the storyline, in which Jack, an actor, appears in a dog food commercial, but this portion of the story was cut from the final version of the film. The standee does show up later in the film, however, when Jack stands next to it as the baby's mother comes to reclaim her child. Snopes contends that the figure in the first scene looks smaller from its appearance in the latter scene because of the distance and angle of the shot, and because the curtains obscure its outstretched arms. As for the contention that a boy died in the house, all the indoor scenes in the film were shot on a Toronto soundstage, and no residential dwellings were used for interior filming